What could the reason that many different “providers” do offer such NextCloud images? Do they contain eva-versions of plug-ins?

The main “official” version of the Nextcloud VM is the one I linked. Then there are many others that forked that repo or made their own installation scripts due to their own preference. Maybe some people do is as a way of learning (I did a few years back), and some people maybe want to use “cutting-edge” dependencies, and some people may have other reasons.

There are equally amount of opinions as there are people and everyone doesn’t like exactly the same stuff.